U.S. patent application number 14/516616 was filed with the patent office on 2015-02-05 for photoelectric conversion element, photoelectric conversion system, and method for production of photoelectric conversion element.
This patent application is currently assigned to Kabushiki Kaisha Toshiba. The applicant listed for this patent is Kabushiki Kaisha Toshiba. Invention is credited to Hironori ASAI, Mizunori EZAKI, Rei HASHIMOTO, Shinya NUNOUE, Shinji SAITO.
Application Number | 20150034968 14/516616 |
Document ID | / |
Family ID | 50099444 |
Filed Date | 2015-02-05 |
United States Patent
Application |
20150034968 |
Kind Code |
A1 |
SAITO; Shinji ; et
al. |
February 5, 2015 |
PHOTOELECTRIC CONVERSION ELEMENT, PHOTOELECTRIC CONVERSION SYSTEM,
AND METHOD FOR PRODUCTION OF PHOTOELECTRIC CONVERSION ELEMENT
Abstract
A photoelectric conversion element of an embodiment is a
photoelectric conversion element which performs photoelectric
conversion by receiving illumination light having n light emission
peaks having a peak energy Ap (eV) (where 1.ltoreq.p.ltoreq.n and
2.ltoreq.n) of 1.59.ltoreq.Ap.ltoreq.3.26 and a full width at half
maximum Fp (eV) (where 1.ltoreq.p.ltoreq.n and 2.ltoreq.n), wherein
the photoelectric conversion element includes m photoelectric
conversion layers having a band gap energy Bq (eV) (where
1.ltoreq.q.ltoreq.m and 2.ltoreq.m.ltoreq.n), and the m
photoelectric conversion layers each satisfy the relationship of
Ap-Fp<Bq.ltoreq.Ap with respect to any one of the n light
emission peaks.
Inventors: |
SAITO; Shinji; (Kanagawa,
JP) ; HASHIMOTO; Rei; (Tokyo, JP) ; EZAKI;
Mizunori; (Kanagawa, JP) ; NUNOUE; Shinya;
(Chiba, JP) ; ASAI; Hironori; (Kanagawa,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Kabushiki Kaisha Toshiba |
Minato-ku |
|
JP |
|
|
Assignee: |
Kabushiki Kaisha Toshiba
Minato-ku
JP
|
Family ID: |
50099444 |
Appl. No.: |
14/516616 |
Filed: |
October 17, 2014 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
13953029 |
Jul 29, 2013 |
8896076 |
|
|
14516616 |
|
|
|
|
Current U.S.
Class: |
257/76 ; 438/72;
438/94 |
Current CPC
Class: |
H01L 33/30 20130101;
H01L 31/022475 20130101; H01L 31/024 20130101; Y02P 70/50 20151101;
H01L 31/00 20130101; H01L 31/1848 20130101; H01L 31/03044 20130101;
H01L 31/1852 20130101; Y02P 70/521 20151101; H01L 31/022483
20130101; H01L 33/0075 20130101; H01L 31/03048 20130101; H01L
31/1856 20130101; H01L 31/06875 20130101; H01L 31/167 20130101;
Y02E 10/544 20130101; H01L 31/1884 20130101; H01L 31/02327
20130101 |
Class at
Publication: |
257/76 ; 438/72;
438/94 |
International
Class: |
H01L 31/167 20060101
H01L031/167; H01L 31/18 20060101 H01L031/18; H01L 31/0232 20060101
H01L031/0232; H01L 31/0224 20060101 H01L031/0224; H01L 31/0304
20060101 H01L031/0304; H01L 31/024 20060101 H01L031/024 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 20, 2012 |
JP |
2012-181377 |
Jul 8, 2013 |
JP |
2013-143022 |
Claims
1. A photoelectric conversion system comprising: an illuminator
having n (n is a natural number) light emission peaks each having a
peak energy Ap (eV) (where 1.ltoreq.p.ltoreq.n and 2.ltoreq.n) of
1.59.ltoreq.Ap.ltoreq.3.26 and a full width at half maximum Fp (eV)
(where 1.ltoreq.p.ltoreq.n and 2.ltoreq.n); and an electronic
device having a photoelectric conversion element, the element
includes m photoelectric conversion layers having a band gap energy
Bq (eV) (where 1.ltoreq.q.ltoreq.m and 2.ltoreq.m.ltoreq.n), and
the m (m is a natural number) photoelectric conversion layers each
satisfy the relationship of Ap=Fp<Bq.ltoreq.Ap with respect to
any one of the n light emission peaks.
2. The system according to claim 1, wherein m=n.
3. The system according to claim 1, wherein the m photoelectric
conversion layers each satisfy the relationship of Ap-0.9
Fp<Bq<Ap-0.7 Fp with respect to any one of the n light
emission peaks.
4. The system according to claim 1, wherein the m photoelectric
conversion layers have a composition denoted as
In.sub.xGa.sub.yAl.sub.zN (0<x.ltoreq.1, 0.ltoreq.y<1,
0.ltoreq.z<1 and x+y+z.ltoreq.1).
5. The system according to claim 1, wherein the electronic device
is a portable information terminal.
6. A method for manufacturing a photoelectric conversion element
comprising: forming a GaN-based n-type semiconductor layer on a
substrate; forming on the n-type semiconductor layer a first
photoelectric conversion layer having a composition denoted as
In.sub.x1Ga.sub.y1Al.sub.z1N (0<x1.ltoreq.1, 0.ltoreq.y1<1,
0.ltoreq.z1<1 and x1+y1+z1.ltoreq.1); forming on the first
photoelectric conversion layer a second photoelectric conversion
layer having a composition denoted as In.sub.x2Ga.sub.y2Al.sub.z2N
(0<x2.ltoreq.1, 0.ltoreq.y2<1, 0.ltoreq.z2<1 and
x2+y2+z2.ltoreq.1) wherein x1<x2; forming a GaN-based p-type
semiconductor layer on the second photoelectric conversion layer;
forming a p-side electrode on the p-type semiconductor layer;
removing the substrate to expose the n-type semiconductor layer;
and forming an n-side electrode on the n-type semiconductor layer
on a side opposite to the first photoelectric conversion layer.
7. The method according to claim 6, wherein a heat dissipation
layer of copper (Cu) is formed on the p-side electrode.
8. The method according to claim 6, wherein a reflection layer of
silver (Ag) is formed between the p-type semiconductor layer and
the p-side electrode.
9. The method according to claim 6, wherein the n-type
semiconductor layer, the first photoelectric conversion layer, the
second photoelectric conversion layer and the p-type semiconductor
layer are formed by a MOCVD method.
10. A method for manufacturing a photoelectric conversion element
comprising: forming a GaN-based first p-type semiconductor layer on
a substrate; forming on the first p-type semiconductor layer a
first photoelectric conversion layer having a composition denoted
as In.sub.x1Ga.sub.y1Al.sub.z1N (0<x1.ltoreq.1,
0.ltoreq.y1<1, 0.ltoreq.z1<1 and x1+y1+z1.ltoreq.1); forming
on the first photoelectric conversion layer a second photoelectric
conversion layer having a composition denoted as
In.sub.x2Ga.sub.y2Al.sub.z2N (0<x2.ltoreq.1, 0.ltoreq.y2<1,
0.ltoreq.z2<1 and x2+y2+z2.ltoreq.1) wherein x1<x2; forming a
GaN-based n-type semiconductor layer on the second photoelectric
conversion layer; forming an n-side electrode on the n-type
semiconductor layer; removing the substrate to expose the first
p-type semiconductor layer; forming a second p-type semiconductor
layer on the first p-type semiconductor layer on a side opposite to
the first photoelectric conversion layer; and forming a p-side
electrode on the second p-type semiconductor layer on a side
opposite to the first photoelectric conversion layer.
11. A method for manufacturing a photoelectric conversion element
comprising: forming a GaN-based p-type semiconductor layer on a
substrate; forming on the p-type semiconductor layer a first
photoelectric conversion layer having a composition denoted as
In.sub.x1Ga.sub.y1Al.sub.z1N (0<x1.ltoreq.1, 0.ltoreq.y1<1,
0.ltoreq.z1<1 and x1+y1+z1.ltoreq.1); forming on the first
photoelectric conversion layer a second photoelectric conversion
layer having a composition denoted as In.sub.x2Ga.sub.y2Al.sub.z2N
(0<x2.ltoreq.1, 0.ltoreq.y2<1, 0.ltoreq.z2<1 and
x2+y2+z2.ltoreq.1) wherein x1<x2; forming a GaN-based n-type
semiconductor layer on the second photoelectric conversion layer;
forming an n-side electrode on the n-type semiconductor layer;
removing the substrate to expose the p-type semiconductor layer;
and forming a p-side electrode of ITO or ZnO on the p-type
semiconductor layer on a side opposite to the first photoelectric
conversion layer.
12. The method according to claim 11, wherein the ITO or ZnO is
deposited by a sputtering method.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a division of and claims the benefit of
priority under 35 U.S.C. .sctn.120 from U.S. Ser. No. 13/953,029
filed Jul. 29, 2013, and claims the benefit of priority under 35
U.S.C. .sctn.119 from Japanese Patent Application No. 2012-181377
filed Aug. 20, 2012 and Japanese Patent Application No. 2013-143022
filed Jul. 8, 2013, the entire contents of each of which are
incorporated herein by reference.
FIELD
[0002] Embodiments described herein relate generally to a
photoelectric conversion element, a photoelectric conversion
system, and a method for manufacturing a photoelectric conversion
element.
BACKGROUND
[0003] Photoelectric conversion elements include a variety of types
depending on the structure and application thereof, such as a LED
(Light Emitting Diode), a LD (Laser Diode), a PD (Photo Diode) and
a solar cell. The development of photovoltaic power generation for
generating electric power using a solar cell is actively promoted
as renewable energy is increasingly required.
[0004] The solar cell is essentially structured so as to
efficiently generate electric power by receiving sunlight. On the
other hand, portable information device have come into wide use as
small and multi-functional information devices as represented by
smart phones.
[0005] For example, the required amount of electric power increases
as the portable information device becomes
multi/high-functionalized. However, the capacity of a built-in
battery, such as a lithium battery, which stores electric power is
limited. Therefore, such a problem arises that the frequency of
charging the portable information device is increased.
[0006] Thus, if electric power can be generated by a
high-efficiency solar cell to cover power consumption, the charge
frequency can be reduced, so that a portable information device of
high practicality can be achieved. Particularly, if a photoelectric
conversion element that allows power generation of high efficiency
with indoor illumination light having a light intensity lower than
that of sunlight is achieved, the usefulness of the portable
information device is enhanced.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] FIG. 1 is a sectional schematic view of a photoelectric
conversion element of a first embodiment;
[0008] FIGS. 2A and 2B are views for explaining the structure and
action of the photoelectric conversion element of the first
embodiment;
[0009] FIG. 3 shows a result of determining a relationship between
a peak energy Ap (eV) and a full width at half maximum Fp (eV) of a
light emission peak and a band gap energy Bq (eV) of a
photoelectric conversion layer by simulation;
[0010] FIG. 4 is a sectional schematic view showing a method for
manufacturing a photoelectric conversion element of the first
embodiment;
[0011] FIG. 5 is a sectional schematic view showing the method for
manufacturing a photoelectric conversion element of the first
embodiment;
[0012] FIG. 6 is a sectional schematic view showing the method for
manufacturing a photoelectric conversion element of the first
embodiment;
[0013] FIG. 7 is a sectional schematic view showing the method for
manufacturing a photoelectric conversion element of the first
embodiment;
[0014] FIGS. 8A and 8B are views showing degradation of an InGaN
layer by heating at 1000.degree. C.;
[0015] FIG. 9 is a sectional schematic view of a photoelectric
conversion element of a second embodiment;
[0016] FIG. 10 is a schematic view of a photoelectric conversion
system of a third embodiment;
[0017] FIG. 11 is a sectional schematic view of a photoelectric
conversion element produced in a fifth embodiment;
[0018] FIG. 12 is a sectional schematic view showing a method for
manufacturing a photoelectric conversion element of the fifth
embodiment;
[0019] FIG. 13 is a sectional schematic view showing the method for
manufacturing a photoelectric conversion element of the fifth
embodiment;
[0020] FIG. 14 is a sectional schematic view showing the method for
manufacturing a photoelectric conversion element of the fifth
embodiment;
[0021] FIG. 15 is a sectional schematic view of a photoelectric
conversion element produced in a sixth embodiment; and
[0022] FIG. 16 is a sectional schematic view of a photoelectric
conversion element of a seventh embodiment.
DETAILED DESCRIPTION
[0023] An photoelectric conversion element of an embodiment is a
photoelectric conversion element which performs photoelectric
conversion by receiving illumination light having n (n is a natural
number) light emission peaks having a peak energy Ap (eV) (where
1.ltoreq.p.ltoreq.n and 2.ltoreq.n) of 1.59.ltoreq.Ap.ltoreq.3.26
and a full width at half maximum Fp (eV) (where 1.ltoreq.p.ltoreq.n
and 2.ltoreq.n), wherein the photoelectric conversion element
includes m (m is a natural number) photoelectric conversion layers
having a band gap energy Bq (eV) (where 1.ltoreq.q.ltoreq.m and
2.ltoreq.m.ltoreq.n), and the m photoelectric conversion layers
each satisfy the relationship of Ap-Fp<Bq.ltoreq.Ap with respect
to any one of the n light emission peaks.
[0024] In this specification, a peak energy Ap (eV) or a peak
wavelength .lamda.p (nm) is used when identifying a light emission
peak. The relationship between the former and the latter can be
represented by Ap=1239.8/.lamda.p.
[0025] For example, a peak wavelength of 380 nm corresponds to a
peak energy of 3.26 eV, and a peak wavelength of 780 nm corresponds
to a peak energy of 1.59 eV.
[0026] Embodiments will be described below with reference to the
drawings.
First Embodiment
[0027] A photoelectric conversion element of this embodiment
performs photoelectric conversion by receiving illumination light
having n (n is a natural number) light emission peaks having a peak
energy Ap (eV) (where 1.ltoreq.p.ltoreq.n and 2.ltoreq.n) of
1.59.ltoreq.Ap.ltoreq.3.26 and a full width at half maximum Fp (eV)
(where 1.ltoreq.p.ltoreq.n and 2.ltoreq.n). The photoelectric
conversion element includes m (m is a natural number) photoelectric
conversion layers having a band gap energy Bq (eV) (where
1.ltoreq.q.ltoreq.m and 2.ltoreq.m.ltoreq.n), and the m
photoelectric conversion layers each satisfy the relationship of
Ap-Fp<Bq.ltoreq.Ap with respect to any one of the n light
emission peaks.
[0028] By having the above-described structure, the photoelectric
conversion element of this embodiment can achieve photoelectric
conversion of high efficiency, i.e. power generation of high
efficiency under illumination light having light emission peaks of
a plurality of wavelengths within visible light, like illumination
light emitted from, for example, an illuminator using a LED (Light
Emitting Diode).
[0029] The photoelectric conversion element of this embodiment is
structured to adapt to illumination light emitted from, for
example, a white light illuminator including a blue LED, of which
the wavelength .lamda..sub.1 of the light emission peak is 450 nm
(A.sub.1=2.76 eV), and a yellow LED, of which the wavelength
.lamda..sub.2 of the light emission peak is 560 nm (A.sub.2=2.21
eV). That is, the photoelectric conversion element is a
photoelectric conversion element which performs photoelectric
conversion by receiving illumination light having two light
emission peaks: a first light emission peak having a peak energy
A.sub.1 of 2.76 eV and a second light emission peak having a peak
energy A.sub.2 of 2.21 eV.
[0030] Here, the first light emission peak has a full width at half
maximum F.sub.1 (eV), and the second light emission peak has a full
width at half maximum F.sub.2 (eV).
[0031] The photoelectric conversion element of this embodiment
includes an n-side electrode formed on a side at which light is
incident; a GaN-based n-type semiconductor layer formed below the
n-side electrode; a first photoelectric conversion layer formed
below the n-type semiconductor layer and having a composition
denoted as In.sub.x1Ga.sub.y1Al.sub.z1N (0<x1.ltoreq.1,
0.ltoreq.y1<1, 0.ltoreq.z1<1 and x1+y1+z1.ltoreq.1); a second
photoelectric conversion layer formed below the first photoelectric
conversion layer and having a composition denoted as
In.sub.x2Ga.sub.y2Al.sub.z2N (0<x2.ltoreq.1, 0.ltoreq.y2<1,
0.ltoreq.z2<1 and x2+y2+z2.ltoreq.1) wherein x1<x2; a
GaN-based p-type semiconductor layer formed below the second
photoelectric conversion layer; and a p-side electrode formed below
the p-type semiconductor layer.
[0032] FIG. 1 is a sectional schematic view of the photoelectric
conversion element of this embodiment. In the photoelectric
conversion element of this embodiment, an n-side electrode 10 is
formed on a side at which illumination light shown by white arrows
in the figure is incident (upper side in FIG. 1). The n-side
electrode 10 is a terminal that outputs a current obtained by
photoelectric conversion. The n-side electrode 10 is, for example,
a metal electrode, and can employ a laminated structure of, for
example, Ti (titanium)/Pt (platinum)/Au (gold) from the lower
layer.
[0033] A first n.sup.+-type GaN (gallium nitride) layer 12 is
formed below the n-side electrode 10. A first n.sup.--type GaN
layer 14 is formed below the first n.sup.+-type GaN layer 12. The
first n.sup.+-type GaN layer 12 and the first n.sup.--type GaN
layer 14 contain, for example, Si (silicon) as an n-type
impurity.
[0034] The n-type impurity concentration of the first n.sup.+-type
GaN layer 12 is, for example, 2.times.10.sup.19 atoms/cm.sup.3 to
1.times.10.sup.20 atoms/cm.sup.3. The n-type impurity concentration
of the first n.sup.--type GaN layer 14 is, for example,
1.times.10.sup.18 atoms/cm.sup.3 to 1.times.0.sup.19
atoms/cm.sup.3.
[0035] A first photoelectric conversion layer 16 of, for example,
In.sub.0.15Ga.sub.0.85N (indium gallium nitride) is formed below
the first n.sup.--type GaN layer 14. Here, the first photoelectric
conversion layer 16 has a band gap energy B.sub.1 (eV).
[0036] A first p.sup.--type GaN layer 18 is formed below the first
photoelectric conversion layer 16. A first p.sup.+-type GaN layer
20 is formed below the first p.sup.--type GaN layer 18. The first
p.sup.--type GaN layer 18 and the first p.sup.+-type GaN layer 20
contain, for example, Mg (magnesium) as a p-type impurity.
[0037] The p-type impurity concentration of the first p.sup.--type
GaN layer 18 is, for example, 1.times.10.sup.18 atoms/cm.sup.3 to
1.times.0.sup.19 atoms/cm.sup.3. The p-type impurity concentration
of the first p.sup.+-type GaN layer 20 is, for example,
2.times.10.sup.19 atoms/cm.sup.3 to 1.times.10.sup.20
atoms/cm.sup.3.
[0038] A second n.sup.+-type GaN layer 22 is formed below the first
p.sup.+-type GaN layer 20. A second n.sup.--type GaN layer 24 is
formed below the second n.sup.+-type GaN layer 22. The second
n.sup.+-type GaN layer 22 and the second n.sup.--type GaN layer 24
contain, for example, Si (silicon) as an n-type impurity.
[0039] The n-type impurity concentration of the second n.sup.+-type
GaN layer 22 is, for example, 2.times.10.sup.19 atoms/cm.sup.3 to
1.times.10.sup.20 atoms/cm.sup.3. The n-type impurity concentration
of the second n.sup.--type GaN layer 24 is, for example,
1.times.10.sup.18 atoms/cm.sup.3 to 1.times.0.sup.19
atoms/cm.sup.3.
[0040] A second photoelectric conversion layer 26 of, for example,
In.sub.0.25Ga.sub.0.75N is formed below the second n.sup.--type GaN
layer 24. Here, the second photoelectric conversion layer 26 has a
band gap energy B.sub.2 (eV).
[0041] A second p.sup.--type GaN layer 28 is formed below the
second photoelectric conversion layer 26. A second p.sup.+-type GaN
layer 30 is formed below the second p.sup.--type GaN layer 28. The
second p.sup.--type GaN layer 28 and the second p.sup.+-type GaN
layer 30 contain, for example, Mg (magnesium) as a p-type
impurity.
[0042] The p-type impurity concentration of the second p.sup.--type
GaN layer 28 is, for example, 1.times.10.sup.18 atoms/cm.sup.3 to
1.times.0.sup.19 atoms/cm.sup.3. The p-type impurity concentration
of the second p.sup.+-type GaN layer 30 is, for example,
2.times.10.sup.19 atoms/cm.sup.3 to 1.times.10.sup.20
atoms/cm.sup.3.
[0043] A reflection layer 32 of, for example, a metal is formed
below the second p.sup.+-type GaN layer 30. Incident illumination
light is reflected by the reflection layer of a metal, so that the
efficiency of the photoelectric conversion element is improved.
Particularly, it is desirable to use Ag (silver) as the reflection
layer 32 because it has a high reflectivity to visible light.
[0044] A p-side electrode 34 is formed below the reflection layer
32. The p-side electrode 34 is a terminal that outputs a current
obtained by photoelectric conversion. The p-side electrode 34 is,
for example, a metal electrode, and can employ a laminated
structure of, for example, Au (gold)/Ni (nickel) from the lower
layer.
[0045] A heat dissipation layer 36 of, for example, a metal is
formed below the p-side electrode 34. It is desirable to use Cu
(copper), which has a high heat conductivity, as the heat
dissipation layer 36.
[0046] FIGS. 2A and 2B are views for explaining the structure and
action of the photoelectric conversion element of the this
embodiment. In this embodiment, the photoelectric conversion
element is provided with a photoelectric conversion layer having,
with respect to a peak energy Ap (eV) of a light emission peak of
illumination light, a band gap energy Bq (eV) that is slightly
lower the peak energy Ap (eV).
[0047] Specifically, a first photoelectric conversion layer 16 is
provided which has, with respect to a first light emission peak
having a peak energy A.sub.1 (eV), a band gap energy B.sub.1 (eV)
that is slightly lower than the peak energy A.sub.1 (eV). It is
considered that consequently, as shown in FIG. 2A, the first light
emission peak and the light reception sensitivity curve of the
first photoelectric conversion layer 16 appropriately overlap each
other, so that efficiency of photoelectric conversion by the first
photoelectric conversion layer 16 is improved.
[0048] Similarly, a second photoelectric conversion layer 26 is
provided which has, with respect to a second light emission peak
having a peak energy A.sub.2 (eV), a band gap energy B.sub.2 (eV)
that is slightly lower than the peak energy A.sub.2 (eV). It is
considered that consequently, efficiency of photoelectric
conversion by the second photoelectric conversion layer 26 is
improved.
[0049] The band gap energy B.sub.1 of the first photoelectric
conversion layer 16 is higher than the band gap energy B.sub.2 of
the second photoelectric conversion layer 26. That is,
photoelectric conversion layers are arranged from the side at which
illumination light is incident, in the descending order, with the
highest the first, in terms of the band gap energy Bq (eV).
[0050] By making this arrangement, a portion of high energy in
illumination light is photoelectrically converted by a
photoelectric conversion layer having a high band gap energy.
Therefore, energy loss during photoelectric conversion is
eliminated, so that photoelectric conversion of high efficiency is
achieved.
[0051] For the range of the band gap energy Bq (eV) of the
photoelectric conversion layer, the relationship of
Ap-Fp<Bq.ltoreq.Ap is satisfied with a peak energy Ap (eV) and a
full width at half maximum Fp (eV) of a light emission peak. In
this embodiment, the relationship of
A.sub.1-F.sub.1<B.sub.1.ltoreq.A.sub.1 and
A.sub.2-F.sub.2<B.sub.2.ltoreq.A.sub.2 is satisfied.
[0052] By satisfying this relationship, high photoelectric
conversion efficiency in the photoelectric conversion layer is
achieved. FIG. 2B shows the above-described relationship with the
first light emission peak and the first photoelectric conversion
layer 16 as an example.
[0053] FIG. 3 shows a result of determining a relationship between
a peak energy Ap (eV) and a full width at half maximum Fp (eV) of a
light emission peak and a band gap energy Bq (eV) of a
photoelectric conversion layer by simulation. A band gap energy Bq
(eV) is determined at which photoelectric conversion efficiency is
maximum when the peak energy Ap (eV) of the light emission peak is
fixed at 2.5 eV and the full width at half maximum Fp (eV) is a
variable.
[0054] The plot shows a band gap energy at which photoelectric
conversion efficiency is maximum. As is apparent from the figure,
the relationship of Ap-Fp<Bq.ltoreq.Ap is satisfied for the band
gap energy at which photoelectric conversion efficiency is maximum.
Further, the band gap energy, at which photoelectric conversion
efficiency is maximum, satisfies the relationship of Ap-0.9
Fp<Bq<Ap-0.7 Fp as well. Thus, it is desirable to satisfy the
relationship of Ap-0.9 Fp<Bq<Ap-0.7 Fp for the band gap
energy Bq (eV).
[0055] According to the photoelectric conversion element of this
embodiment, a photoelectric conversion layer having a band gap
energy optimized to a peak energy of a light emission peak of
illumination light is provided, so that incident light energy can
be efficiently converted into electric energy. Particularly, power
generation of high efficiency can be performed in an indoor
environment where an illuminator using a LED is installed.
[0056] Explanations are provided here taking as an example a case
where when the number of light emission peaks is n and the number
of photoelectric conversion layers is m, n and m each are 2, i.e.
m=n. It is desirable to set m=n from the viewpoint of achieving
high efficiency.
[0057] However, the relationship between m and n may be m.noteq.n,
for example as in a case where the number of light emission peaks
is 3 (n=3) and the number of photoelectric conversion layers is 2
(m=2). For example, it is conceivable that 2 photoelectric
conversion layers optimized to 2 light emission peaks having a high
intensity in 3 light emission peaks are provided.
[0058] In this embodiment, the gradient of a plane direction of a
surface with respect to a plane of lamination of a GaN-based
semiconductor layer formed on a single-crystal silicon substrate 40
is, for example, 0 degree or more and 8 degrees or less. In this
case, it is desirable to satisfy 0.1.ltoreq.X1.ltoreq.0.25 in the
first photoelectric conversion layer 16 having a composition
denoted as In.sub.x1Ga.sub.y1Al.sub.z1N (0<x1.ltoreq.1,
0.ltoreq.y1<1, 0.ltoreq.z1<1 and x1+y1+z1.ltoreq.1) and
satisfy 0.15.ltoreq.X2.ltoreq.0.35 in the second photoelectric
conversion layer 26 having a composition denoted as
In.sub.x2Ga.sub.y2Al.sub.z2N (0<x2.ltoreq.1, 0.ltoreq.y2<1,
0.ltoreq.z2<1 and x2+y2+z2.ltoreq.1). By ensuring that the
composition of In is in the above-described range, conversion
efficiency with respect to blue light and yellow light is
optimized.
[0059] However, the gradient of a plane direction of a surface with
respect to a plane of lamination of a GaN-based semiconductor layer
formed on the single-crystal silicon substrate 40 is desirable to
15 degree or more and 65 degrees or less from the viewpoint of
improving photoelectric conversion efficiency. In this case, it is
desirable to satisfy 0.2.ltoreq.X1.ltoreq.0.4 in the first
photoelectric conversion layer 16 having a composition denoted as
In.sub.x1Ga.sub.y1Al.sub.z1N (0<x1, 0.ltoreq.y1<1, 0z1<1
and x1+y1+z1.ltoreq.1) and satisfy 0.25.ltoreq.X2.ltoreq.0.60 in
the second photoelectric conversion layer 26 having a composition
denoted as In.sub.x2Ga.sub.y2Al.sub.z2N (0<x2.ltoreq.1,
0.ltoreq.y2<1, 0.ltoreq.z2<1 and x2+y2+z2.ltoreq.1). By
ensuring that the composition of In is in the above-described
range, conversion efficiency with respect to blue light and yellow
light is optimized.
[0060] Al (aluminum) may be included in the first or second
photoelectric conversion layer 16 or 26. By including Al, the
integrity of lattices or band gap energy can be adjusted. By
including Al, crystallinity can be improved.
[0061] A material having a composition other than the composition
denoted as In.sub.xGa.sub.yAl.sub.zN (0<x.ltoreq.1,
0.ltoreq.y<1, 0.ltoreq.z<1 and x+y+z.ltoreq.1) can also be
applied to the photoelectric conversion layer. However, the
material having the above-described composition is a desirable
material for power generation in indoor illumination using a LED
because photoelectric conversion efficiency in the visible light
region is high.
[0062] A method for manufacturing a photoelectric conversion
element of this embodiment will now be described. The method for
manufacturing a photoelectric conversion element of this embodiment
includes forming a GaN-based n-type semiconductor layer on a
substrate; forming on the n-type semiconductor layer a first
photoelectric conversion layer having a composition denoted as
In.sub.x1Ga.sub.y1Al.sub.z1N (0<x1.ltoreq.1, 0.ltoreq.y1<1,
0.ltoreq.z1<1 and x1+y1+z1.ltoreq.1); forming on the first
photoelectric conversion layer a second photoelectric conversion
layer having a composition denoted as In.sub.x2Ga.sub.y2Al.sub.z2N
(0<x2.ltoreq.1, 0.ltoreq.y2<1, 0.ltoreq.z2<1 and
x2+y2+z2.ltoreq.1) wherein x1<x2; forming a GaN-based p-type
semiconductor layer on the second photoelectric conversion layer;
forming a p-side electrode on the p-type semiconductor layer;
removing the substrate to expose the n-type semiconductor layer;
and forming an n-side electrode on the n-type semiconductor layer
on a side opposite to the first photoelectric conversion layer.
[0063] FIGS. 4, 5, 6 and 7 are sectional schematic views showing
the method for manufacturing a photoelectric conversion element of
this embodiment.
[0064] First, for example, a single-crystal silicon substrate 40
having a thickness of about 500 .mu.m is provided, and carried in a
MOCVD device. Next, an AlN buffer layer 11, a first n.sup.+-type
GaN (gallium nitride) layer 12 and a first n.sup.--type GaN layer
14 are formed on the single-crystal silicon substrate 40 by a MOCVD
(Metal Organic Chemical Vapor Deposition) method.
[0065] The AlN buffer layer 11, the first n.sup.+-type GaN (gallium
nitride) layer 12 and the first n.sup.--type GaN layer 14 are
crystal-grown with TMG (trimethyl gallium) and NH.sub.3 as a raw
material gas, for example, under heated conditions at 1000.degree.
C. to 1100.degree. C. SiH.sub.4 (silane) is used for introduction
of Si as an n-type impurity.
[0066] After formation of the n.sup.--type GaN layer 14, TMI
(trimethyl indium) is added to the raw material gas to form a first
photoelectric conversion layer 16 of, for example,
In.sub.0.15Ga.sub.0.85N (indium gallium nitride).
[0067] After formation of the first photoelectric conversion layer
16, a first p.sup.--type GaN layer 18 and a first p.sup.+-type GaN
layer 20 are formed. The first p.sup.--type GaN layer 18 and the
first p.sup.+-type GaN layer 20 are crystal-grown with TMG
(trimethyl gallium) and NH.sub.3 as a raw material gas under heated
conditions at 1000.degree. C. Cp2Mg (cyclopentadienyl magnesium) is
used for introduction of Mg as a p-type impurity (FIG. 4).
[0068] After formation of the first p.sup.+-type GaN layer 20, a
second n.sup.+-type GaN layer 22, a second n.sup.--type GaN layer
24, a second photoelectric conversion layer 26, a second
p.sup.--type GaN layer 28 and a second p.sup.+-type GaN layer 30
are formed (FIG. 5) by a process similar to the above-described
process for forming the first p.sup.+-type GaN layer 20 from the
first n.sup.+-type GaN (gallium nitride) layer 12.
[0069] The second photoelectric conversion layer 26 is, for
example, In.sub.0.25Ga.sub.0.75N. The second photoelectric
conversion layer 26 contains In (indium) in a concentration higher
than that of the first photoelectric conversion layer 16. That is,
the first photoelectric conversion layer 16 is a photoelectric
conversion layer having a band gap energy greater than that of the
second photoelectric conversion layer 26.
[0070] After formation of the second p.sup.+-type GaN layer 30, the
single-crystal silicon substrate 40 is taken out from the MOCVD
device. A reflection layer 32 and a p-side electrode 34 are formed
on the second p.sup.+-type GaN layer 30.
[0071] The reflection layer 32 is, for example, Ag (silver), and
the p-side electrode 34 is, for example, a laminated film of Ni
(nickel)/Au (gold) from the reflection layer 32. The reflection
layer 32 and the p-side electrode 34 are formed by, for example, an
electron beam vapor deposition method in an electron beam vapor
deposition device.
[0072] Next, a Cu (copper) substrate 36 having a size similar to
that of the single-crystal silicon substrate 40 is provided. For
example a SnAgCu solder (not shown) is deposited on the Cu
substrate 36.
[0073] Thereafter, the Cu substrate 36 and the p-side electrode 34
are superimposed on each other with the SnAgCu solder held
therebetween, and laminated together by heating to, for example,
270.degree. C. while a pressure is applied in a vacuum (FIG.
6).
[0074] Next, the single-crystal silicon substrate 40 is thinned to
a thickness of about 50 .mu.m by, for example, polishing it by a
polishing machine. Thereafter, the single-crystal silicon substrate
40 is removed in its entirety by a dry etching device using a
CF.sub.4 (methane tetrafluoride) gas as a dry etching gas.
Thereafter, the dry etching gas is changed to Cl.sub.2 (chlorine),
and the AlN buffer layer 11 is etched to expose the first
n.sup.+-type GaN (gallium nitride) layer 12 (FIG. 7).
[0075] Thereafter, the surface of the first n.sup.+-type GaN
(gallium nitride) layer 12 is etched with KOH (potassium hydroxide)
to form an unevenness on the surface. After a rinsing treatment, an
n-side electrode 10 is formed on the surface of the first
n.sup.+-type GaN (gallium nitride) layer 12. The n-side electrode
10 is, for example, a laminated film of Ti (titanium)/Pt
(platinum)/Au (gold) from, for example, the first n.sup.+-type GaN
(gallium nitride) layer 12 side.
[0076] By the above step, the photoelectric conversion element
shown in FIG. 1 can be formed.
[0077] FIGS. 8A and 8B are views showing degradation of an InGaN
layer by heating at 1000.degree. C. A sample is irradiated with
ultraviolet rays, a fluorescence is observed, and degradation of
crystallinity is evaluated by light emission unevenness.
[0078] FIG. 8A shows a state before heating, and FIG. 8B shows a
state after heating. Uniform light emission is observed before
heating, whereas light emission unevenness is significant after
heating, and it is apparent that the crystallinity of the InGaN
layer is degraded. As a result of studies by the present
disclosure, it has been found that degradation of characteristics
is significant particularly in the case of a high concentration of
In (indium) (high composition).
[0079] It has become evident that degradation depends on the
heating temperature and heating time, and an InGaN layer having a
higher In composition is degraded at a lower temperature and in a
shorter time. When the crystallinity of the InGaN layer is
degraded, the photoelectric conversion efficiency of the
photoelectric conversion element is deteriorated.
[0080] According to this embodiment, the first photoelectric
conversion layer 16 as an InGaN layer having a low In composition
is first formed, and the second photoelectric conversion layer 26
as an InGaN layer having a high In composition is then formed.
Therefore, degradation of the photoelectric conversion layer by the
heat treatment during film formation can be suppressed to achieve
high photoelectric conversion efficiency.
[0081] It has been found that when a p-side electrode is formed on
a p-type GaN layer, the crystallinity of the surface of the p-side
GaN layer is important. That is, if the crystallinity of the
surface of the p-side GaN layer is disordered, the contact
resistance of the p-side electrode increases particularly when the
p-side electrode is a metal. Therefore, such a problem arises that
particularly when processing with accelerated particles, such as
dry etching, or polishing processing is carried out for surface
processing of the p-side GaN layer, the contact resistance is
significantly increased.
[0082] According to this embodiment, it is required to carry out
dry etching or polishing processing for removing the substrate on
the p-type GaN layer, or the like, but by using the present
structure, the p-side electrode can be formed without carrying out
such processing. Therefore, a p-side electrode having a low contact
resistance can be achieved. Accordingly, a photoelectric conversion
element with a low resistance loss and high efficiency can be
achieved.
[0083] In place of a single-crystal silicon substrate, any other
substrate capable of forming a semiconductor layer on the substrate
can also be used.
Second Embodiment
[0084] A photoelectric conversion element of this embodiment is
same as that of the first embodiment except that the photoelectric
conversion element includes three photoelectric conversion layers.
Therefore, descriptions of matters that are in common with the
first embodiment are omitted.
[0085] The photoelectric conversion element of this embodiment is
structured to adapt to illumination light emitted from, for
example, a white light illuminator including a blue LED, of which
the wavelength .lamda..sub.1 of the light emission peak is 450 nm
(A.sub.1=2.76 eV), a green LED, of which the wavelength
.lamda..sub.2 of the light emission peak is 520 nm (A.sub.2=2.38
eV), and a red LED, of which the wavelength .lamda..sub.3 of the
light emission peak is 630 nm (A.sub.2=1.97 eV). That is, the
photoelectric conversion element is a photoelectric conversion
element which performs photoelectric conversion by receiving
illumination light having three light emission peaks: a first light
emission peak having a peak energy A.sub.1 of 2.76 eV, a second
light emission peak having a peak energy A.sub.2 of 2.38 eV and a
third light emission peak having a peak energy A.sub.2 of 1.97
eV.
[0086] Here, the first light emission peak has a full width at half
maximum F.sub.1 (eV), the second light emission peak has a full
width at half maximum F.sub.2 (eV), and the third light emission
peak has a full width at half maximum F.sub.3 (eV).
[0087] The photoelectric conversion element of this embodiment
includes an n-side electrode formed on a side at which illumination
light is incident; a GaN-based n-type semiconductor layer formed
below the n-side electrode; a first photoelectric conversion layer
formed below the n-type semiconductor layer and having a
composition denoted as In.sub.x1Ga.sub.y1Al.sub.z1N
(0<x1.ltoreq.1, 0.ltoreq.y1<1, 0.ltoreq.z1<1 and
x1+y1+z1.ltoreq.1); a second photoelectric conversion layer formed
below the first photoelectric conversion layer and having a
composition denoted as In.sub.x2Ga.sub.y2Al.sub.z2N
(0<x2.ltoreq.1, 0.ltoreq.y2<1, 0.ltoreq.z2<1 and
x2+y2+z2.ltoreq.1) wherein x1<x2; a third photoelectric
conversion layer formed below the second photoelectric conversion
layer and having a composition denoted as
In.sub.x3Ga.sub.y3Al.sub.z3N (0<x3.ltoreq.1, 0.ltoreq.y3<1,
0.ltoreq.z3<1 and x3+y3+z3.ltoreq.1) wherein x2<x3; a
GaN-based p-type semiconductor layer formed below the third
photoelectric conversion layer; and a p-side electrode formed below
the p-type semiconductor layer.
[0088] FIG. 9 is a sectional schematic view of a photoelectric
conversion element of this embodiment. In the photoelectric
conversion element of this embodiment, an n-side electrode 10 is
formed on a side at which illumination light shown by white arrows
in the figure is incident (upper side in FIG. 1).
[0089] A first n.sup.+-type GaN (gallium nitride) layer 12 is
formed below the n-side electrode 10. A first n.sup.--type GaN
layer 14 is formed below the first n.sup.+-type GaN layer 12. The
first n.sup.+-type GaN layer 12 and the first n.sup.--type GaN
layer 14 contain, for example, Si (silicon) as an n-type
impurity.
[0090] A first photoelectric conversion layer 16 of, for example,
In.sub.0.15Ga.sub.0.85N (indium gallium nitride) is formed below
the first n.sup.--type GaN layer 14. Here, the first photoelectric
conversion layer 16 has a band gap energy B.sub.1 (eV).
[0091] A first p.sup.--type GaN layer 18 is formed below the first
photoelectric conversion layer 16. A first p.sup.+-type GaN layer
20 is formed below the first p.sup.--type GaN layer 18. The first
p.sup.--type GaN layer 18 and the first p.sup.+-type GaN layer 20
contain, for example, Mg (magnesium) as a p-type impurity.
[0092] A second n.sup.+-type GaN layer 22 is formed below the first
p.sup.+-type GaN layer 20. A second n.sup.--type GaN layer 24 is
formed below the second n.sup.+-type GaN layer 22. The second
n.sup.+-type GaN layer 22 and the second n.sup.--type GaN layer 24
contain, for example, Si (silicon) as an n-type impurity.
[0093] A second photoelectric conversion layer 26 of, for example,
In.sub.0.20Ga.sub.0.80N is formed below the second n.sup.--type GaN
layer 24. Here, the second photoelectric conversion layer 26 has a
band gap energy B.sub.2 (eV).
[0094] A second p.sup.--type GaN layer 28 is formed below the
second photoelectric conversion layer 26. A second p.sup.+-type GaN
layer 30 is formed below the second p.sup.--type GaN layer 28. The
second p.sup.--type GaN layer 28 and the second p.sup.+-type GaN
layer 30 contain, for example, Mg (magnesium) as a p-type
impurity.
[0095] A third n.sup.+-type GaN layer 42 is formed below the second
p.sup.+-type GaN layer 30. A third n.sup.--type GaN layer 44 is
formed below the third n.sup.+-type GaN layer 42. The third
n.sup.+-type GaN layer 42 and the third n.sup.--type GaN layer 44
contain, for example, Si (silicon) as an n-type impurity.
[0096] A third photoelectric conversion layer 46 of, for example,
In.sub.0.30Ga.sub.0.70N is formed below the third n.sup.--type GaN
layer 44. Here, the third photoelectric conversion layer 46 has a
band gap energy B.sub.3 (eV).
[0097] A third p.sup.--type GaN layer 48 is formed below the third
photoelectric conversion layer 46. A third p.sup.+-type GaN layer
50 is formed below the third p.sup.--type GaN layer 48. The third
p.sup.--type GaN layer 48 and the third p.sup.+-type GaN layer 50
contain, for example, Mg (magnesium) as a p-type impurity.
[0098] A reflection layer 32 of, for example, a metal is formed
below the third p.sup.+-type GaN layer 50. Incident illumination
light is reflected by the reflection layer of a metal, so that the
efficiency of the photoelectric conversion element is improved.
Particularly, it is desirable to use Ag (silver) as the reflection
layer 32 because it has a high reflectivity to visible light.
[0099] A p-side electrode 34 is formed below the reflection layer
32. The p-side electrode 34 is a terminal that outputs a current
obtained by photoelectric conversion.
[0100] A heat dissipation layer 36 of, for example, a metal is
formed below the p-side electrode 34.
[0101] The band gap energy B.sub.1 of the first photoelectric
conversion layer 16 is higher than the band gap energy B.sub.2 of
the second photoelectric conversion layer 26, and the band gap
energy B.sub.2 of the second photoelectric conversion layer 26 is
higher than the band gap energy B.sub.3 of the third photoelectric
conversion layer 46. That is, photoelectric conversion layers are
arranged from the side at which illumination light is incident, in
the descending order, with the highest the first, in terms of the
band gap energy Bq (eV).
[0102] By making this arrangement, a portion of high energy in
illumination light is photoelectrically converted by a
photoelectric conversion layer having a high band gap energy.
Therefore, energy loss during photoelectric conversion is
eliminated, so that photoelectric conversion of high efficiency is
achieved.
[0103] For the range of the band gap energy Bq (eV) of the
photoelectric conversion layer, the relationship of
Ap-Fp<Bq.ltoreq.Ap is satisfied with a peak energy Ap (eV) and a
full width at half maximum Fp (eV) of a light emission peak. By
satisfying this relationship, high photoelectric conversion
efficiency in the photoelectric conversion layer is achieved. In
this embodiment, the relationship of
A.sub.1-F.sub.1<B.sub.1.ltoreq.A.sub.1,
A.sub.2-F.sub.2<B.sub.2.ltoreq.A.sub.2 and
A.sub.3-F.sub.3<B.sub.3.ltoreq.A.sub.3 is satisfied.
[0104] According to the photoelectric conversion element of this
embodiment, three photoelectric conversion layers having a band gap
energy optimized to the peak energy of three light emission peaks
of illumination light are provided. Therefore, incident light
energy can be efficiently converted. Particularly, power generation
of high efficiency can be performed in an indoor environment where
an illuminator using a LED is installed.
[0105] The photoelectric conversion element of this embodiment can
be produced by a method similar to the production method described
in the first embodiment.
Third Embodiment
[0106] A photoelectric conversion system of this embodiment
includes an illuminator having n light emission peaks having a peak
energy Ap (eV) (where 1.ltoreq.p.ltoreq.n and 2.ltoreq.n) of
1.59.ltoreq.Ap.ltoreq.3.26 and a full width at half maximum Fp (eV)
(where 1.ltoreq.p.ltoreq.n and 2.ltoreq.n); and an electronic
device having a photoelectric conversion element, wherein the
photoelectric conversion element includes m photoelectric
conversion layers having a band gap energy Bq (eV) (where
1.ltoreq.q.ltoreq.m and 2.ltoreq.m.ltoreq.n), and the m
photoelectric conversion layers each satisfy the relationship of
Ap-Fp<Bq.ltoreq.Ap with respect to any one of the n light
emission peaks. The structure of the photoelectric element is
similar to that in the first embodiment. Therefore, descriptions of
matters that are in common with the first embodiment are
omitted.
[0107] FIG. 10 is a schematic view of the photoelectric conversion
system of this embodiment. The photoelectric conversion system of
this embodiment includes an illuminator 60 and an electronic device
70.
[0108] The illuminator 60 is, for example, an illuminator in an
office or a factory, which uses a LED (Light Emitting Diode). The
illuminator is, for example, a white light illuminator including a
blue LED, of which the wavelength .lamda..sub.1 of the light
emission peak is 450 nm (A.sub.1=2.76 eV), and a yellow LED, of
which the wavelength .lamda..sub.2 of the light emission peak is
560 nm (A.sub.2=2.21 eV).
[0109] The electronic device 70 is, for example, a portable
information device such as a smart phone. The electronic device 70
includes a photoelectric conversion element 72. The photoelectric
conversion element 72 photoelectrically converts illumination light
from the illuminator 60 to generate electric power.
[0110] The photoelectric conversion element 72 is structured to
adapt to illumination light emitted from, for example, a white
light illuminator including a blue LED, of which the wavelength
.lamda..sub.1 of the light emission peak is 450 nm (A.sub.1=2.76
eV), and a yellow LED, of which the wavelength .lamda..sub.2 of the
light emission peak is 560 nm (A.sub.2=2.21 eV). That is, the
photoelectric conversion element is a photoelectric conversion
element which performs photoelectric conversion by receiving
illumination light having two light emission peaks: a first light
emission peak having a peak energy A.sub.1 of 2.76 eV and a second
light emission peak having a peak energy A.sub.2 of 2.21 eV.
[0111] Here, the first light emission peak has a full width at half
maximum F.sub.1 (eV), and the second light emission peak has a full
width at half maximum F.sub.2 (eV). For the range of the band gap
energy Bq (eV) of the photoelectric conversion layer, the
relationship of Ap-Fp<Bq.ltoreq.Ap is satisfied with a peak
energy Ap (eV) and a full width at half maximum Fp (eV) of a light
emission peak. In this embodiment, the relationship of
A.sub.1-F.sub.1<B.sub.1.ltoreq.A.sub.1 and
A.sub.2-F.sub.2<B.sub.2.ltoreq.A.sub.2 is satisfied.
[0112] According to this embodiment, a high-efficiency
photoelectric conversion system can be achieved by optimizing the
light emission characteristics of the illuminator and the
photoelectric conversion element characteristics of the electronic
device. Therefore, power generation of high efficiency with indoor
illumination light having a light intensity lower than that of
sunlight is possible and for example, the usefulness of the
portable information device is enhanced.
[0113] An example of this embodiment will be described below. The
illuminator 60 has light emission peaks of four different
wavelengths. The illuminator 60 may become an illuminator having
the best efficiency by adjusting full width at half maximum of
light emission peaks of LED and a range of wavelength of visible
light. Peaks of the emission wavelength are 450 nm, 535 nm, 590 nm,
605 nm (2.57 eV, 2.32 eV, 2.10 eV, 2.05 eV). Good color rendering
characteristics and high efficiency can be achieved by making full
width at half maximum of the peaks 20 nm. Theoretically, 390 lm/W
can be achieved. Band gaps of an photoelectric conversion element
72 which receives light from the illuminator 60 will be 2.57 eV,
2.32 eV, 2.10 eV, 2.05 eV which is same as that of the emission
layers of the illuminator 60. Because they have same band gaps,
same materials can be used for the emission layers of the
illuminator 60 and the photoelectric conversion layers of the
photoelectric conversion element 72. More preferably, band gaps of
the photoelectric conversion layers of the photoelectric conversion
element 72 are 2.66 eV, 2.25 eV, 2.04 eV, 2.00 eV. With the band
gaps, more efficient power generation will be possible. This is due
to relation between a shape of emission spectrum and a shape of
absorption spectrum. In Gallium Nitride semiconductor material, a
quantum confined stark effect (QCSE) which is a narrowing of band
gap from inherent band gap due to internal electric field in the
material will occur when InGaN layers are used for emission layers
or photoelectric conversion layers. In this case, a wave length of
emission becomes shorter than that of absorption by a screening
effect due to carrier injection. In InGaN material, a band tail
level may be generated due to micro-scale high In concentration
regions generated in the material which are caused by low mixing
ability of In in GaN. In LED, recombination of carriers in this
band tail level may be low level injection. In this case, a peak of
emission light will be at longer side. Such various phenomena may
be observed in the photoelectric conversion element 72. In
conclusion, high efficient photoelectric conversion element can be
achieved by making the range of the band gap energy Bq (eV) of the
photoelectric conversion layer of the photoelectric conversion
element to satisfy the relationship of Ap-Fp<Bq.ltoreq.Ap, more
preferably Ap-0.9 Fp<Bq<Ap-0.7 Fp, with a peak energy Ap (eV)
and a full width at half maximum Fp (eV) of a light emission peak
of the illuminator.
Fourth Embodiment
[0114] A method for manufacturing a photoelectric conversion
element of this embodiment is same as that of the first embodiment
except that a semiconductor layer on a substrate is formed by a DC
sputtering method rather than a MOCVD method. Therefore,
descriptions of matters that are in common with the first
embodiment are omitted. The structure of the photoelectric element
is similar to that in the first embodiment.
[0115] The production method of this embodiment will be described
with reference to FIGS. 4, 5, 6 and 7.
[0116] First, for example, a single-crystal silicon substrate 40
having a thickness of about 500 .mu.m is provided, and carried in a
DC sputtering device . Next, an AlN buffer layer 11, a first
n.sup.+-type GaN (gallium nitride) layer 12 and a first
n.sup.--type GaN layer 14 are formed on the single-crystal silicon
substrate 40 by a DC sputtering method.
[0117] The AlN buffer layer 11, the first n.sup.+-type GaN (gallium
nitride) layer 12 and the first n.sup.--type GaN layer 14 are
crystal-grown with a GaN sintered body as a raw material, for
example, under heated conditions at 800.degree. C. to 900.degree.
C. Si is used for an n-type impurity.
[0118] After formation of the n.sup.--type GaN layer 14, a GaN
sintered body and an InN sintered body are co-sputtered to form a
first photoelectric conversion layer 16 of, for example,
In.sub.0.15Ga.sub.0.85N (indium gallium nitride).
[0119] After formation of the first photoelectric conversion layer
16, a first p.sup.--type GaN layer 18 and a first p.sup.+-type GaN
layer 20 are formed. The first p.sup.--type GaN layer 18 and the
first p.sup.+-type GaN layer 20 are crystal-grown under heated
conditions at 800.degree. C. to 900.degree. C. using as a raw
material a GaN sintered body containing Mg (FIG. 4).
[0120] After formation of the first p.sup.+-type GaN layer 20, a
second n.sup.+-type GaN layer 22, a second n.sup.--type GaN layer
24, a second photoelectric conversion layer 26, a second
p.sup.--type GaN layer 28 and a second p.sup.+-type GaN layer 30
are formed (FIG. 5) by a process similar to the above-described
process for forming the first p.sup.+-type GaN layer 20 from the
first n.sup.+-type GaN (gallium nitride) layer 12.
[0121] The second photoelectric conversion layer 26 is, for
example, In.sub.0.25Ga.sub.0.75N. The second photoelectric
conversion layer 26 contains In (indium) in a concentration higher
than that of the first photoelectric conversion layer 16. That is,
the first photoelectric conversion layer 16 is a photoelectric
conversion layer having a band gap energy greater than that of the
second photoelectric conversion layer 26.
[0122] After formation of the second p.sup.+-type GaN layer 30, the
single-crystal silicon substrate is taken out from the DC
sputtering device. A reflection layer 32 and a p-side electrode 34
are formed on the second p.sup.+-type GaN layer 30. Subsequent
steps are same as those in the first embodiment (FIGS. 6 and
7).
[0123] According to this embodiment, a high-efficiency
photoelectric conversion element can be produced by a more
inexpensive method as compared to the first embodiment.
Fifth Embodiment
[0124] A method for manufacturing a photoelectric conversion
element of this embodiment includes forming a GaN-based first
p-type semiconductor layer on a substrate; forming on the first
p-type semiconductor layer a first photoelectric conversion layer
having a composition denoted as In.sub.x1Ga.sub.y1Al.sub.z1N
(0<x1.ltoreq.1, 0.ltoreq.y1<1, 0.ltoreq.z1<1 and
x1+y1+z1.ltoreq.1); forming on the first photoelectric conversion
layer a second photoelectric conversion layer having a composition
denoted as In.sub.x2Ga.sub.y2Al.sub.z2N (0<x2.ltoreq.1,
0.ltoreq.y2<1, 0.ltoreq.z2<1 and x2+y2+z2.ltoreq.1) wherein
x1<x2; forming a GaN-based n-type semiconductor layer on the
second photoelectric conversion layer; forming an n-side electrode
on the n-type semiconductor layer; removing the substrate to expose
the first p-type semiconductor layer; further forming a second
p-type semiconductor layer on the first p-type semiconductor layer
on a side opposite to the first photoelectric conversion layer; and
forming a p-side electrode on the second p-type semiconductor layer
on a side opposite to the first photoelectric conversion layer.
[0125] FIG. 11 is a sectional schematic view of the photoelectric
conversion element produced in this embodiment. It is different
from the photoelectric conversion element of the first embodiment
shown in FIG. 1 in that the p-side electrode is formed on a side at
which illumination light is incident.
[0126] That is, the photoelectric conversion element of this
embodiment is structured to adapt to illumination light emitted
from, for example, a white light illuminator including a blue LED,
of which the wavelength .lamda..sub.1 of the light emission peak is
450 nm (A.sub.1=2.76 eV), and a yellow LED, of which the wavelength
.lamda..sub.2 of the light emission peak is 560 nm (A.sub.2=2.21
eV). That is, the photoelectric conversion element is a
photoelectric conversion element which performs photoelectric
conversion by receiving illumination light having two light
emission peaks: a first light emission peak having a peak energy
A.sub.1 of 2.76 eV and a second light emission peak having a peak
energy A.sub.2 of 2.21 eV.
[0127] Here, the first light emission peak has a full width at half
maximum F.sub.1 (eV), and the second light emission peak has a full
width at half maximum F.sub.2 (eV).
[0128] In the photoelectric conversion element of this embodiment,
an p-side electrode 34 is formed on a side at which illumination
light shown by white arrows in the figure is incident (upper side
in FIG. 11). The p-side electrode 34 is a terminal that outputs a
current obtained by photoelectric conversion. The p-side electrode
34 is, for example, a metal electrode, and can employ, for example,
a laminated structure of Ni (nickel)/Au (gold) from the
semiconductor layer side.
[0129] A first p.sup.+-type GaN layer 20 is formed below the p-side
electrode 34. A first p.sup.--type GaN layer 18 is formed below the
first p.sup.+-type GaN layer 20. The first p.sup.+-type GaN layer
20 and the first p.sup.--type GaN layer 18 contain, for example, Mg
(magnesium) as a p-type impurity.
[0130] The p-type impurity concentration of the first p.sup.--type
GaN layer 18 is, for example, 1.times.10.sup.18 atoms/cm.sup.3 to
1.times.0.sup.19 atoms/cm.sup.3. The p-type impurity concentration
of the first p.sup.+-type GaN layer 20 is, for example,
2.times.10.sup.19 atoms/cm.sup.3 to 1.times.10.sup.20
atoms/cm.sup.3.
[0131] A first photoelectric conversion layer 16 of, for example,
In.sub.0.15Ga.sub.0.85N (indium gallium nitride) is formed below
the first p.sup.--type GaN layer 18. Here, the first photoelectric
conversion layer 16 has a band gap energy B.sub.1 (eV).
[0132] A first n.sup.--type GaN layer 14 is formed below the first
photoelectric conversion layer 16. A first n.sup.+-type GaN layer
12 is formed below the first n.sup.--type GaN layer 14. The first
n.sup.+-type GaN layer 12 and the first n.sup.--type GaN layer 14
contain, for example, Si (silicon) as an n-type impurity.
[0133] The n-type impurity concentration of the first n.sup.+-type
GaN layer 12 is, for example, 2.times.10.sup.19 atoms/cm.sup.3 to
1.times.10.sup.20 atoms/cm.sup.3. The n-type impurity concentration
of the first n.sup.--type GaN layer 14 is, for example,
1.times.10.sup.18 atoms/cm.sup.3 to 1.times.0.sup.19
atoms/cm.sup.3.
[0134] A second p.sup.+-type GaN layer 30 is formed below the first
n.sup.+-type GaN layer 12. A second p.sup.--type GaN layer 28 is
formed below the second p.sup.+-type GaN layer 30. The second
p.sup.+-type GaN layer 30 and the second p.sup.--type GaN layer 28
contain, for example, Mg (magnesium) as a p-type impurity.
[0135] The p-type impurity concentration of the second p.sup.--type
GaN layer 28 is, for example, 1.times.10.sup.18 atoms/cm.sup.3 to
1.times.0.sup.19 atoms/cm.sup.3. The p-type impurity concentration
of the second p.sup.+-type GaN layer 30 is, for example,
2.times.10.sup.19 atoms/cm.sup.3 to 1.times.10.sup.20
atoms/cm.sup.3.
[0136] A second photoelectric conversion layer 26 of, for example,
In.sub.0.25Ga.sub.0.75N is formed below the second p.sup.--type GaN
layer 28. Here, the second photoelectric conversion layer 26 has a
band gap energy B.sub.2 (eV).
[0137] A second n.sup.--type GaN layer 24 is formed below the
second photoelectric conversion layer 26. A second n.sup.+-type GaN
layer 22 is formed below the second n.sup.--type GaN layer 24. The
second n.sup.+-type GaN layer 22 and the second n.sup.--type GaN
layer 24 contain, for example, Si (silicon) as an n-type
impurity.
[0138] The n-type impurity concentration of the second n.sup.+-type
GaN layer 22 is, for example, 2.times.10.sup.19 atoms/cm.sup.3 to
1.times.10.sup.20 atoms/cm.sup.3. The n-type impurity concentration
of the second n.sup.--type GaN layer 24 is, for example,
1.times.10.sup.18 atoms/cm.sup.3 to 1.times.0.sup.19
atoms/cm.sup.3.
[0139] A reflection layer 32 of, for example, a metal is formed
below the second n.sup.+-type GaN layer 22. Incident illumination
light is reflected by the reflection layer of a metal, so that the
efficiency of the photoelectric conversion element is improved.
Particularly, it is desirable to use Ag (silver) as the reflection
layer 32 because it has a high reflectivity to visible light.
[0140] An n-side electrode 10 is formed below the reflection layer
32. The n-side electrode 10 is a terminal that outputs a current
obtained by photoelectric conversion. The n-side electrode 10 is,
for example, a metal electrode, and can employ, for example, a
laminated structure of Ti (titanium)/Pt (platinum)/Au (gold) from
the reflection layer 32 side.
[0141] A heat dissipation layer 36 of, for example, a metal is
formed below the n-side electrode 10. It is desirable to use Cu
(copper), which has a high heat conductivity, as the heat
dissipation layer 36. Particularly when the photoelectric
conversion element is used in such a form as so called a
concentrating solar cell with an optical condenser, a material
having a high heat conductivity is desirable because the heat
quantity per unit area is large. It may be aluminum in terms of
costs, or by using a diamond substrate, although costs are somewhat
increased, reduction of efficiency and degradation by heat can be
suppressed even if the optical concentration ratio is further
increased.
[0142] The band gap energy B.sub.1 of the first photoelectric
conversion layer 16 is higher than the band gap energy B.sub.2 of
the second photoelectric conversion layer 26. That is,
photoelectric conversion layers are arranged from the side at which
illumination light is incident, in the descending order, with the
highest the first, in terms of the band gap energy Bq (eV).
[0143] By making this arrangement, a portion of high energy in
illumination light is photoelectrically converted by a
photoelectric conversion layer having a high band gap energy.
Therefore, energy loss during photoelectric conversion is
eliminated, so that photoelectric conversion of high efficiency is
achieved.
[0144] For the range of the band gap energy Bq (eV) of the
photoelectric conversion layer, the relationship of
Ap-Fp<Bq.ltoreq.Ap is satisfied with a peak energy Ap (eV) and a
full width at half maximum Fp (eV) of a light emission peak. In
this embodiment, the relationship of
A.sub.1-F.sub.1<B.sub.1.ltoreq.A.sub.1 and
A.sub.2-F.sub.2<B.sub.2.ltoreq.A.sub.2 is satisfied.
[0145] By satisfying this relationship, high photoelectric
conversion efficiency in the photoelectric conversion layer is
achieved.
[0146] According to the photoelectric conversion element produced
in this embodiment, a photoelectric conversion layer having a band
gap energy optimized to a light emission peak of illumination light
is provided, so that incident light energy can be efficiently
converted. Particularly, power generation of high efficiency can be
performed in an indoor environment where an illuminator using a LED
is installed.
[0147] A method for manufacturing a photoelectric conversion
element of this embodiment will now be described. FIGS. 12, 13 and
14 are sectional schematic views showing the method for
manufacturing a photoelectric conversion element of this
embodiment.
[0148] First, for example, a single-crystal silicon substrate 40
having a thickness of about 500 .mu.m is provided, and carried in a
MOCVD device. Next, an AlN buffer layer 11, a first p.sup.+-type
GaN layer 20 and a first p.sup.--type GaN layer 18 are formed on
the single-crystal silicon substrate 40 by a MOCVD (Metal Organic
Chemical Vapor Deposition) method.
[0149] The AlN buffer layer 11, the first p.sup.+-type GaN layer 20
and the first p.sup.--type GaN layer 18 are crystal-grown with TMG
(trimethyl gallium) and NH.sub.3 as a raw material gas, for
example, under heated conditions at 1000.degree. C. to 1100.degree.
C. Cp2Mg (cyclopentadienyl magnesium) is used for introduction of
Mg as a p-type impurity.
[0150] After formation of the first p.sup.--type GaN layer 18, TMI
(trimethyl indium) is added to the raw material gas to form a first
photoelectric conversion layer 16 of, for example,
In.sub.0.15Ga.sub.0.85N (indium gallium nitride).
[0151] After formation of the first photoelectric conversion layer
16, a first n.sup.--type GaN layer 14 and a first n.sup.+-type GaN
(gallium nitride) layer 12 are formed. The first n.sup.+-type GaN
(gallium nitride) layer 12 and the first n.sup.--type GaN layer 14
are crystal-grown with TMG (trimethyl gallium) and NH.sub.3 as a
raw material gas, for example, under heated conditions at
1000.degree. C. to 1100.degree. C. SiH.sub.4 (silane) is used for
introduction of Si as an n-type impurity.
[0152] After formation of the first n.sup.+-type GaN (gallium
nitride) layer 12, a second p.sup.+-type GaN layer 30, a second
p.sup.--type GaN layer 28, a second photoelectric conversion layer
26, a second n.sup.--type GaN layer 24 and a second n.sup.+-type
GaN layer 22 are formed (FIG. 12) by a process similar to the
above-described process for forming the first n.sup.+-type GaN
(gallium nitride) layer 12 from the first p.sup.+-type GaN layer
20.
[0153] The second photoelectric conversion layer 26 is, for
example, In.sub.0.25Ga.sub.0.75N. The second photoelectric
conversion layer 26 contains In (indium) in a concentration higher
than that of the first photoelectric conversion layer 16. That is,
the first photoelectric conversion layer 16 is a photoelectric
conversion layer having a band gap energy greater than that of the
second photoelectric conversion layer 26.
[0154] After formation of the second n.sup.+-type GaN layer 22, the
single-crystal silicon substrate 40 is taken out from the MOCVD
device. A reflection layer 32 and an n-side electrode 10 are formed
on the second n.sup.+-type GaN layer 22.
[0155] The reflection layer 32 is, for example, Ag (silver), and
the n-side electrode 10 is, for example, a laminated film of Ti
(titanium)/Pt (platinum)/Au (gold) from the reflection layer 32
side. The reflection layer 32 and the n-side electrode 10 are
formed by, for example, an electron beam vapor deposition method in
an electron beam vapor deposition device.
[0156] Next, a Cu (copper) substrate 36 having a size similar to
that of the single-crystal silicon substrate 40 is provided. For
example a SnAgCu solder (not shown) is deposited on the Cu
substrate 36.
[0157] Thereafter, the Cu substrate 36 and the n-side electrode 10
are superimposed on each other with the SnAgCu solder held
therebetween, and laminated together by heating to, for example,
270.degree. C. while a pressure is applied in a vacuum (FIG.
13).
[0158] Next, the single-crystal silicon substrate 40 is thinned to
a thickness of about 50 .mu.m by, for example, polishing it by a
polishing machine. Thereafter, the single-crystal silicon substrate
40 is removed in its entirety by a dry etching device using a
CF.sub.4 (methane tetrafluoride) gas as a dry etching gas.
Thereafter, the dry etching gas is changed to Cl.sub.2 (chlorine),
and the AlN buffer layer 11 is etched to expose the first
p.sup.+-type GaN layer 20 (FIG. 14).
[0159] Thereafter, a third p.sup.+-type GaN layer (not shown:
second p-type semiconductor layer) is formed on the first
p.sup.+-type GaN layer 20 by, for example, a MOCVD method.
Thereafter, a p-side electrode 34 is formed on the surface of the
third p.sup.+-type GaN layer. The p-side electrode 34 is, for
example, a metal electrode, and can employ, for example, a
laminated structure of Ni (nickel)/Au (gold) from the semiconductor
layer side.
[0160] By the above step, the photoelectric conversion element
shown in FIG. 11 can be produced.
[0161] According to this embodiment, the first photoelectric
conversion layer 16 as an InGaN layer having a low In composition
is first formed, and the second photoelectric conversion layer 26
as an InGaN layer having a high In composition is then formed.
Therefore, degradation of the photoelectric conversion layer can be
suppressed to achieve high photoelectric conversion efficiency.
[0162] It has been found that when a p-side electrode is formed on
a p-type GaN layer, the crystallinity of the surface of the p-side
GaN layer is important as described above. According to this
embodiment, the single-crystal silicon substrate 40 is peeled off
from the first p.sup.+-type GaN layer 20, and thereafter further a
third p.sup.+-type GaN layer (second p-type semiconductor layer) is
formed by a MOCVD method.
[0163] In this way, a p-side electrode 34 is formed on the surface
of the third p.sup.+-type GaN layer without carrying out processing
using accelerated particles, such as dry etching, or polishing
processing. Therefore, the p-side electrode 34 can be formed while
the disorder of crystallinity of the surface of the p-type GaN
layer is reduced. Therefore, a p-side electrode having a low
contact resistance can be achieved. Accordingly, a photoelectric
conversion element with a low resistance loss and high efficiency
can be produced.
Sixth Embodiment
[0164] A method for manufacturing a photoelectric conversion
element of this embodiment includes forming a GaN-based p-type
semiconductor layer on a substrate; forming on the p-type
semiconductor layer a first photoelectric conversion layer having a
composition denoted as In.sub.x1Ga.sub.y1Al.sub.z1N
(0<x1.ltoreq.1, 0.ltoreq.y1<1, 0.ltoreq.z1<1 and
x1+y1+z1.ltoreq.1); forming on the first photoelectric conversion
layer a second photoelectric conversion layer having a composition
denoted as In.sub.x2Ga.sub.y2Al.sub.z2N (0<x2.ltoreq.1,
0.ltoreq.y2<1, 0.ltoreq.z2<1 and x2+y2+z2.ltoreq.1) wherein
x1<x2; forming a GaN-based n-type semiconductor layer on the
second photoelectric conversion layer; forming an n-side electrode
on the n-type semiconductor layer; removing the substrate to expose
the p-type semiconductor layer; and forming a p-side electrode of
ITO (indium tin oxide) or ZnO (zinc oxide) on the p-type
semiconductor layer on a side opposite to the first photoelectric
conversion layer.
[0165] The method for manufacturing a photoelectric conversion
element of this embodiment is same as that in the fifth embodiment
except that the p-side electrode of ITO or ZnO is formed without
forming the second p-type semiconductor layer. Therefore,
descriptions of matters that are in common with the fifth
embodiment are omitted.
[0166] FIG. 15 is a sectional schematic view of the photoelectric
conversion element produced in this embodiment. It is different
from the photoelectric conversion element of the fifth embodiment
shown in FIG. 11 in that a p-side electrode 80 formed on a first
p.sup.+-type GaN layer 20 is a transparent semiconductor electrode
of ITO or ZnO.
[0167] The method for manufacturing a photoelectric conversion
element is same as that in the fifth embodiment up to the step of
exposing the first p.sup.+-type GaN layer 20 (FIG. 14).
[0168] Subsequently, a p-side electrode of ITO (indium tin oxide)
or ZnO (zinc oxide) is formed by, for example, a sputtering
method.
[0169] For example, in the case of ITO, the electrode is formed by
RF sputtering with ITO as a target. For example, ITO is deposited
in an oxygen atmosphere at a RF power of 200 W at a substrate
temperature of 25.degree. C.
[0170] For example, by carrying out annealing in an oxygen
atmosphere at 400.degree. C. for 3 minutes after deposition of ITO,
both the transparency and electrical conductivity of ITO can be
secured.
[0171] According to this embodiment, the first photoelectric
conversion layer 16 as an InGaN layer having a low In composition
is first formed, and the second photoelectric conversion layer 26
as an InGaN layer having a high In composition is then formed.
Therefore, degradation of the photoelectric conversion layer can be
suppressed to achieve high photoelectric conversion efficiency.
[0172] ITO and ZnO are n-type semiconductors. The interface between
ITO or ZnO and the first p.sup.+-type GaN layer 20 (p-type
semiconductor layer) forms a tunnel junction, so that the contact
resistance can be reduced.
[0173] Therefore, a p-side electrode having a low contact
resistance can be achieved, and a photoelectric conversion element
with a low resistance loss and high efficiency can be produced.
Seventh Embodiment
[0174] A photoelectric conversion element includes an n-side
electrode formed on a side at which illumination light is incident;
a GaN-based n-type semiconductor layer formed below the electrode;
a first photoelectric conversion layer formed below the
semiconductor layer and having a composition denoted as
In.sub.x1Ga.sub.y1Al.sub.z1N (0<x1.ltoreq.1, 0.ltoreq.y1<1,
0.ltoreq.z1<1 and x1+y1+z1.ltoreq.1); a second photoelectric
conversion layer formed below the photoelectric conversion layer
and having a composition denoted as In.sub.x2Ga.sub.y2Al.sub.z2N
(0<x2.ltoreq.1, 0.ltoreq.y2<1, 0.ltoreq.z2<1 and
x2+y2+z2.ltoreq.1) wherein x1<x2; a third photoelectric
conversion layer formed below the second photoelectric conversion
layer and having a composition denoted as
In.sub.x3Ga.sub.y3Al.sub.z3N (0<x3.ltoreq.1, 0.ltoreq.y3<1,
0.ltoreq.z3<1 and x3+y3+z3.ltoreq.1) wherein x2<x3; a
GaN-based p-type semiconductor layer formed below the third
photoelectric conversion layer; and a p-side electrode formed below
the p-type semiconductor layer.
[0175] The photoelectric conversion element of this embodiment has
the above-described structure, so that high photoelectric
conversion efficiency can be achieved by an easy production method
in, for example, a photoelectric conversion element that generates
electric power with sunlight as incident light.
[0176] A method for manufacturing a photoelectric conversion
element of this embodiment includes forming a GaN-based n-type
semiconductor layer on a substrate; forming on the n-type
semiconductor layer a first photoelectric conversion layer having a
composition denoted as In.sub.x1Ga.sub.y1Al.sub.z1N
(0<x1.ltoreq.1, 0.ltoreq.y1<1, 0.ltoreq.z1<1 and
x1+y1+z1.ltoreq.1); forming on the first photoelectric conversion
layer a second photoelectric conversion layer having a composition
denoted as In.sub.x2Ga.sub.y2Al.sub.z2N (0<x2.ltoreq.1,
0.ltoreq.y2<1, 0.ltoreq.z2<1 and x2+y2+z2.ltoreq.1) wherein
x1<x2; forming on the second photoelectric conversion layer a
third photoelectric conversion layer having a composition denoted
as In.sub.x3Ga.sub.y3Al.sub.z3N (0<x3.ltoreq.1,
0.ltoreq.y3<1, 0.ltoreq.z3<1 and x3+y3+z3.ltoreq.1) wherein
x2<x3; forming a GaN-based p-type semiconductor layer on the
third photoelectric conversion layer; forming a p-side electrode on
the p-type semiconductor layer; removing the substrate to expose
the n-type semiconductor layer; and forming an n-side electrode on
the n-type semiconductor layer on a side opposite to the first
photoelectric conversion layer.
[0177] The method for manufacturing a photoelectric conversion
element of this embodiment can enhance the photoelectric conversion
efficiency of, for example, a photoelectric conversion element that
generates electric power with sunlight as incident light.
[0178] FIG. 16 is a sectional schematic view of the photoelectric
conversion element of this embodiment. In the photoelectric
conversion element of this embodiment, an n-side electrode 10 is
formed on a side at which sunlight shown by white arrows in the
figure is incident (upper side in FIG. 16). The n-side electrode 10
is, for example, a metal electrode, and can employ a laminated
structure of, for example, Ti (titanium)/Pt (platinum)/Au (gold)
from the lower layer.
[0179] A first n.sup.+-type GaN (gallium nitride) layer 12 is
formed below the n-side electrode 10. A first n.sup.--type GaN
layer 14 is formed below the first n.sup.+-type GaN layer 12. The
first n.sup.+-type GaN layer 12 and the first n.sup.--type GaN
layer 14 contain, for example, Si (silicon) as an n-type
impurity.
[0180] A first photoelectric conversion layer 16 of, for example,
In.sub.0.25Ga.sub.0.75N (indium gallium nitride) is formed below
the first n.sup.--type GaN layer 14. Here, the first photoelectric
conversion layer 16 has a band gap energy B.sub.1 (eV).
[0181] A first p.sup.--type GaN layer 18 is formed below the first
photoelectric conversion layer 16. A first p.sup.+-type GaN layer
20 is formed below the first p.sup.--type GaN layer 18. The first
p.sup.--type GaN layer 18 and the first p.sup.+-type GaN layer 20
contain, for example, Mg (magnesium) as a p-type impurity.
[0182] A second n.sup.+-type GaN layer 22 is formed below the first
p.sup.+-type GaN layer 20. A second n.sup.--type GaN layer 24 is
formed below the second n.sup.+-type GaN layer 22. The second
n.sup.+-type GaN layer 22 and the second n.sup.--type GaN layer 24
contain, for example, Si (silicon) as an n-type impurity.
[0183] A second photoelectric conversion layer 26 of, for example,
In.sub.0.40Ga.sub.0.60N is formed below the second n.sup.--type GaN
layer 24. Here, the second photoelectric conversion layer 26 has a
band gap energy B.sub.2 (eV).
[0184] A second p.sup.--type GaN layer 28 is formed below the
second photoelectric conversion layer 26. A second p.sup.+-type GaN
layer 30 is formed below the second p.sup.--type GaN layer 28. The
second p.sup.--type GaN layer 28 and the second p.sup.+-type GaN
layer 30 contain, for example, Mg (magnesium) as a p-type
impurity.
[0185] A third n.sup.+-type GaN layer 42 is formed below the second
p.sup.+-type GaN layer 30. A third n.sup.--type GaN layer 44 is
formed below the third n.sup.+-type GaN layer 42. The third
n.sup.+-type GaN layer 42 and the third n.sup.--type GaN layer 44
contain, for example, Si (silicon) as an n-type impurity.
[0186] A third photoelectric conversion layer 46 of, for example,
In.sub.0.45Ga.sub.0.55N is formed below the third n.sup.--type GaN
layer 44. Here, the third photoelectric conversion layer 46 has a
band gap energy B.sub.3 (eV).
[0187] A third p.sup.--type GaN layer 48 is formed below the third
photoelectric conversion layer 46. A third p.sup.+-type GaN layer
50 is formed below the third p.sup.--type GaN layer 48. The third
p.sup.--type GaN layer 48 and the third p.sup.+-type GaN layer 50
contain, for example, Mg (magnesium) as a p-type impurity.
[0188] A p-side electrode 90 of, for example, ITO (indium tin
oxide) is formed below the third p.sup.+-type GaN layer 50. The
p-side electrode 90 is a terminal that outputs a current obtained
by photoelectric conversion.
[0189] A Si (silicon) substrate 100 is provided below the p-side
electrode 90. The Si (silicon) substrate 100 functions as a
photoelectric conversion layer. The structure of the Si substrate
that functions as a photoelectric conversion layer is same as the
structure of a normal single-crystal silicon substrate solar cell,
and includes an n.sup.+Si layer, an nSi layer, a pSi layer, a
p.sup.+Si layer and a p-side electrode. For the electrode, the ITO
part may be drawn out from the side face and operated independently
and in this case, it is necessary to provide different circuits for
the Si substrate and GaN at a location to which the electrode is
drawn out even from the ITO electrode, so that the structure is
complicated. The electrode may be drawn out from the p-side
electrode of the Si substrate 100, rather than from ITO. In this
case, operations are stabilized by designing the power generation
amperage of each layer so that the currents become equal.
[0190] In this embodiment, carrier collection efficiency is
improved by providing on the p-type semiconductor layer side a
layer corresponding to so called a back surface electric field
layer and providing on the n-type semiconductor layer side a layer
corresponding to so called a window layer, with first, second and
third photoelectric conversion layers held therebetween,
respectively.
[0191] The band gap energy B.sub.1 of the first photoelectric
conversion layer 16 is higher than the band gap energy B.sub.2 of
the second photoelectric conversion layer 26, and the band gap
energy B.sub.2 of the second photoelectric conversion layer 26 is
higher than the band gap energy B.sub.3 of the third photoelectric
conversion layer 46. That is, photoelectric conversion layers are
arranged from the side at which illumination light is incident, in
the descending order, with the highest the first, in terms of the
band gap energy Bq (eV).
[0192] By making this arrangement, a portion of high energy in
illumination light is photoelectrically converted by a
photoelectric conversion layer having a high band gap energy.
Therefore, energy loss during photoelectric conversion is
eliminated, so that photoelectric conversion of high efficiency is
achieved.
[0193] A method for manufacturing a photoelectric conversion
element of this embodiment will now be described.
[0194] First, for example, a single-crystal silicon substrate
having a thickness of about 500 .mu.m is provided, and carried in a
MOCVD device. Next, an AlN buffer layer (not shown), a first
n.sup.+-type GaN (gallium nitride) layer 12 and a first
n.sup.--type GaN layer 14 are formed on the single-crystal silicon
substrate 40 by a MOCVD (Metal Organic Chemical Vapor Deposition)
method.
[0195] The AlN buffer layer, the first n.sup.+-type GaN (gallium
nitride) layer 12 and the first n.sup.--type GaN layer 14 are
crystal-grown with TMG (trimethyl gallium) and NH.sub.3 as a raw
material gas, for example, under heated conditions at 1000.degree.
C. to 1100.degree. C. SiH.sub.4 (silane) is used for introduction
of Si as an n-type impurity.
[0196] After formation of the n.sup.--type GaN layer 14, TMI
(trimethyl indium) is added to the raw material gas to form a first
photoelectric conversion layer 16 of, for example,
In.sub.0.25Ga.sub.0.75N (indium gallium nitride).
[0197] After formation of the first photoelectric conversion layer
16, a first p.sup.--type GaN layer 18 and a first p.sup.+-type GaN
layer 20 are formed. The first p.sup.--type GaN layer 18 and the
first p.sup.+-type GaN layer 20 are crystal-grown with TMG
(trimethyl gallium) and NH.sub.3 as a raw material gas under heated
conditions at 1000.degree. C. Cp2Mg (cyclopentadienyl magnesium) is
used for introduction of Mg as a p-type impurity.
[0198] After formation of the first p.sup.+-type GaN layer 20, a
second n.sup.+-type GaN layer 22, a second n.sup.--type GaN layer
24, a second photoelectric conversion layer 26, a second
p.sup.--type GaN layer 28 and a second p.sup.+-type GaN layer 30
are formed by a process similar to the above-described process for
forming the first p.sup.+-type GaN layer 20 from the first
n.sup.+-type GaN (gallium nitride) layer 12.
[0199] The second photoelectric conversion layer 26 is, for
example, In.sub.0.40Ga.sub.0.60N. The second photoelectric
conversion layer 26 contains In (indium) in a concentration higher
than that of the first photoelectric conversion layer 16. That is,
the first photoelectric conversion layer 16 is a photoelectric
conversion layer having a band gap energy greater than that of the
second photoelectric conversion layer 26.
[0200] After formation of the second p.sup.+-type GaN layer 30, a
third n.sup.+-type GaN layer 42, a third n.sup.--type GaN layer 44,
a third photoelectric conversion layer 46, a third p.sup.--type GaN
layer 48 and a third p.sup.+-type GaN layer 50 are formed by a
process similar to the above-described process for forming the
first p.sup.+-type GaN layer 20 from the first n.sup.+-type GaN
(gallium nitride) layer 12.
[0201] The third photoelectric conversion layer 46 is, for example,
In.sub.0.45Ga.sub.0.55N. The third photoelectric conversion layer
46 contains In (indium) in a concentration higher than that of the
second photoelectric conversion layer 26. That is, the second
photoelectric conversion layer 26 is a photoelectric conversion
layer having a band gap energy greater than that of the third
photoelectric conversion layer 46.
[0202] After formation of the third p.sup.+-type GaN layer 50, the
single-crystal silicon substrate is taken out from the MOCVD
device. A p-side electrode 90 of ITO (indium tin oxide) is formed
on the third p.sup.+-type GaN layer 50. Formation of the p-side
electrode 90 is carried out by, for example, an electron beam vapor
deposition method in an electron beam vapor deposition device.
[0203] Next, a silicon substrate having a size similar to that of
the single-crystal silicon substrate is provided. For example an
ITO film is deposited on the silicon substrate.
[0204] Thereafter, ITO of the p-side electrode of the
single-crystal silicon substrate and the ITO film of the silicon
substrate are superimposed on each other so as to contact each
other and laminated together by heating to, for example,
270.degree. C. while a pressure is applied in a vacuum.
[0205] Next, the single-crystal silicon substrate used for growth
of the GaN-based semiconductor is thinned to a thickness of about
50 .mu.m by, for example, polishing it by a polishing machine.
Thereafter, the single-crystal silicon substrate 40 is removed in
its entirety by a dry etching device using a CF.sub.4 (methane
tetrafluoride) gas as a dry etching gas. Thereafter, the dry
etching gas is changed to Cl.sub.2 (chlorine), and the AlN buffer
layer is etched to expose the first n.sup.+-type GaN (gallium
nitride) layer 12.
[0206] Thereafter, the surface of the first n.sup.+-type GaN
(gallium nitride) layer 12 is etched with KOH (potassium hydroxide)
to form an unevenness on the surface. After a rinsing treatment, an
n-side electrode 10 is formed on the surface of the first
n.sup.+-type GaN (gallium nitride) layer 12. The n-side electrode
10 is, for example, a laminated film of Ti (titanium)/Pt
(platinum)/Au (gold) from, for example, the first n.sup.+-type GaN
(gallium nitride) layer 12 side.
[0207] By the above step, the photoelectric conversion element
shown in FIG. 16 can be formed.
[0208] As described above, as a result of studies by the present
disclosure, it has been found that degradation of characteristics
is significant particularly in the case of a high concentration of
In (indium) (high composition).
[0209] It has become evident that degradation depends on the
heating temperature and heating time, and an InGaN layer having a
higher In composition is degraded at a lower temperature and in a
shorter time. When the crystallinity of the InGaN layer is
degraded, the photoelectric conversion efficiency of the
photoelectric conversion element is deteriorated.
[0210] According to this embodiment, the first photoelectric
conversion layer 16 as an InGaN layer having a low In composition
is first formed, and the second photoelectric conversion layer 26
as an InGaN layer having a high In composition is then formed.
Further, the third photoelectric conversion layer 46, an InGaN
layer having a higher In composition, is formed. Therefore,
degradation of the photoelectric conversion layer by the heat
treatment during film formation can be suppressed to achieve high
photoelectric conversion efficiency.
[0211] It has been found that when a p-side electrode is formed on
a p-type GaN layer, the crystallinity of the surface of the p-side
GaN layer is important. That is, if the crystallinity of the
surface of the p-side GaN layer is disordered, the contact
resistance of the p-side electrode increases. Therefore,
particularly when processing with accelerated particles, such as
dry etching, or polishing processing is carried out for surface
processing of the p-side GaN layer, the contact resistance is
significantly increased.
[0212] According to this embodiment, the p-side electrode can be
formed without carrying out dry etching or polishing processing for
removing the substrate on the p-type GaN layer, or the like.
Therefore, a p-side electrode having a low contact resistance can
be achieved. Accordingly, a photoelectric conversion element with a
low resistance loss and high efficiency can be achieved.
[0213] By conforming the order of lamination of the layers of the
photoelectric conversion element to the configuration of this
embodiment shown in FIG. 16, it becomes easy to form the p-side
electrode without carrying out dry etching or polishing processing
for removing the substrate on the p-type GaN layer, or the
like.
[0214] In this embodiment, explanations have been provided taking
as an example a case where the photoelectric conversion layer of
the InGaN layer has three layers, but a photoelectric conversion
layer of an InGaN layer having two or four layers can also be
provided in consideration of production costs and photoelectric
conversion efficiency.
[0215] The Si substrate that functions as a photoelectric
conversion layer may not necessarily be provided. When the Si
substrate is not provided, the composition of the photoelectric
conversion layer can also be set to, for example,
In.sub.0.40Ga.sub.0.60N In.sub.0.55Ga.sub.0.45N and InN in the
ascending order, with the lowest the first, in terms of the In
composition, for enhancing efficiency of absorption of light having
a long wavelength.
[0216] In place of a single-crystal silicon substrate, any other
substrate capable of forming a semiconductor layer on the substrate
can also be used.
[0217] In the explanation of the embodiments, descriptions have
been omitted for parts and the like that are not directly required
for explanation of the present disclosure, in the photoelectric
conversion element, the photoelectric conversion system, the method
for manufacturing a photoelectric conversion element, and so on,
but required components that are related to the photoelectric
conversion element, the photoelectric conversion system and the
method for manufacturing a photoelectric conversion element can
also be appropriately selected and used.
[0218] For example, layers that facilitate characteristics and the
production method can also be appropriately inserted between the
substrate and the semiconductor layer, the electrode or the
like.
[0219] While certain embodiments have been described, these
embodiments have been presented by way of example only, and are not
intended to limit the scope of the inventions. Indeed, the
photoelectric conversion element, the photoelectric conversion
system and the method for manufacturing a photoelectric conversion
element described herein may be embodied in a variety of other
forms; furthermore, various omissions, substitutions and changes in
the form of the devices and methods described herein may be made
without departing from the spirit of the inventions. The
accompanying claims and their equivalents are intended to cover
such forms or modifications as would fall within the scope and
spirit of the inventions.
* * * * *